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In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either a quantum of energy from a crystal lattice vibration (phonon) or by long-range interactions such as spin fluctuations. The novel superconductor UTe2 is a rare material wherein electrons form pairs in a unique spin-triplet state with potential topological properties. Theoretically, spin-triplet superconductivity in UTe2 may be explained in terms of pairing mediated by either ferromagnetic or antiferromagnetic fluctuations. Experimentally, the magnetic properties of UTe2 remain enigmatic. Here we report muon spin rotation/relaxation (μSR) measurements on independently grown UTe2 single crystals that exhibit either a single or double phase transition in the specific heat near the onset of superconductivity. In the absence of an applied magnetic field, we observe an inhomogeneous distribution of magnetic fields in a sizeable volume fraction of all samples studied. The growth in the volume of the magnetic regions is halted by the onset of superconductivity at the critical temperature Tc. Upon further cooling, slow fluctuations of the local fields persist until a disordered spin frozen state appears below about one tenth of Tc. The μSR results are consistent with the formation of magnetic clusters in UTe2 due to the influence of disorder on long-range electronic correlations or geometrical magnetic frustration associated with the ladder-like U sublattice structure. Our findings suggest that inhomogeneous magnetic clusters are responsible for the ubiquitous residual linear term and low-temperature upturn in the temperature dependence of the specific heat in UTe2 below Tc. The omnipresent magnetic inhomogeneity has implications for the interpretation of other low-temperature experimental observations in the superconducting state of UTe2.

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe$_2$ is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe$_2$ are unclear. By way of muon spin rotation/relaxation ($\mu$SR) measurements on independently grown UTe$_2$ single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat ($C$) and the low-temperature upturn in $C/T$ versus $T$. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe$_2$., Comment: 33 pages, 9 figures

CeCd$_3$As$_3$ is a rare-earth triangular-lattice antiferromagnet with large inter-layer separation. Our field-dependent heat capacity measurements at dilution fridge temperatures allow us to trace the field-evolution of the spin-excitation gaps throughout the antiferromagnetic and paramagnetic regions. The distinct gap evolution places strong constraints on the microscopic pseudo-spin model, which, in return, yields a close {\it quantitative} description of the gap behavior. This analysis provides crucial insights into the nature of the magnetic state of CeCd$_3$As$_3$, with a certainty regarding its stripe order and low-energy model parameters that sets a compelling paradigm for exploring and understanding the rapidly growing family of the rare-earth-based triangular-lattice systems., Main Text: 6 pages, 3 figures SM: 19 pages, 23 figures Some clarifications are made relative to previous version

Using magnetic force microscopy operating at sub-Kelvin temperatures we characterize the heavy-fermion superconductor CeCoIn$_5$. We pinpoint the absolute London penetration depth $\lambda(0) = 435 \pm 20$ nm and report its temperature dependence, which is closely linked to the symmetry of the superconducting gap. In addition, we directly measure the pinning force of individual Abrikosov vortices and estimate the critical current density $j_c = 9 \times 10^4$ A/cm$^2$. In contrast to the related, well-established tunnel diode oscillator technique, our method is capable of resolving inhomogeneities $locally$ on the micrometer-scale at ultra-low temperature.

The $d$-wave superconductor ${\mathrm{CeCoIn}}_{5}$ has been proposed as a strong candidate for supporting the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state near the low-temperature boundary of its upper critical field. Neutron diffraction, however, finds spin-density-wave (SDW) order in this part of the phase diagram for field in the $a\text{\ensuremath{-}}b$ plane, and evidence for the SDW disappears as the applied field is rotated toward the tetragonal $c$ axis. It is important to understand the interplay between the SDW and a possible FFLO state in ${\mathrm{CeCoIn}}_{5}$, as the mere existence of an SDW does not necessarily exclude an FFLO state. Here, based on a model constructed on the basis of available experiments, we show that an FFLO state competes with an SDW phase. The SDW state in ${\mathrm{CeCoIn}}_{5}$ is stabilized when the field is directed close to the $a\text{\ensuremath{-}}b$ plane. When the field is rotated toward the $c$ axis, the FFLO state emerges, and the SDW phase disappears. In the FFLO state, the nodal planes with extra quasiparticles (where the superconducting order parameter is zero) are perpendicular to the field, and in the SDW phase, the quasiparticle density of states is reduced. We test this model prediction by measuring heat transported by normal quasiparticles in the superconducting state. As a function of field, we observe a reduction of thermal conductivity for field close to the $a\text{\ensuremath{-}}b$ plane and an enhancement of thermal conductivity when field is close to the $c$ axis, consistent with theoretical expectations. Our modeling and experiments, therefore, indicate the existence of the FFLO state when field is parallel to the $c$ axis.

The d-wave superconductor CeCoIn_{5} has been proposed as a strong candidate for supporting the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state near the low-temperature boundary of its upper critical field. Neutron diffraction, however, finds spin-density-wave (SDW) order in this part of the phase diagram for field in the a-b plane, and evidence for the SDW disappears as the applied field is rotated toward the tetragonal c axis. It is important to understand the interplay between the SDW and a possible FFLO state in CeCoIn_{5}, as the mere existence of an SDW does not necessarily exclude an FFLO state. Here, based on a model constructed on the basis of available experiments, we show that an FFLO state competes with an SDW phase. The SDW state in CeCoIn_{5} is stabilized when the field is directed close to the a-b plane. When the field is rotated toward the c axis, the FFLO state emerges, and the SDW phase disappears. In the FFLO state, the nodal planes with extra quasiparticles (where the superconducting order parameter is zero) are perpendicular to the field, and in the SDW phase, the quasiparticle density of states is reduced. We test this model prediction by measuring heat transported by normal quasiparticles in the superconducting state. As a function of field, we observe a reduction of thermal conductivity for field close to the a-b plane and an enhancement of thermal conductivity when field is close to the c axis, consistent with theoretical expectations. Our modeling and experiments, therefore, indicate the existence of the FFLO state when field is parallel to the c axis.

Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice, exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.

Kitaev magnets are materials with bond-dependent Ising interactions between localized spins on a honeycomb lattice. Such interactions could lead to a quantum spin-liquid (QSL) ground state at zero temperature. Recent theoretical studies suggest two potential signatures of a QSL at finite temperatures, namely, a scaling behavior of thermodynamic quantities in the presence of quenched disorder, and a two-step release of the magnetic entropy. Here, we present both signatures in ${\mathrm{Ag}}_{3}{\mathrm{LiIr}}_{2}{\mathrm{O}}_{6}$ which is synthesized from $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ by replacing the interlayer Li atoms with Ag atoms. In addition, the dc susceptibility data confirm the absence of a long-range order, and the ac susceptibility data rule out a spin-glass transition. These observations suggest a closer proximity to the QSL in ${\mathrm{Ag}}_{3}{\mathrm{LiIr}}_{2}{\mathrm{O}}_{6}$ compared to its parent compound $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ that orders at 15 K. We discuss an enhanced spin-orbit coupling due to a mixing between silver $d$ and oxygen $p$ orbitals as a potential underlying mechanism.

We present inelastic neutron scattering data on the quantum paramagnet AgVOAsO4 that establish the system is a S=1/2 alternating spin chain compound and provide a direct measurement of the spin gap. We also present experimental evidence for two different types of field-induced magnetic order between mu_0H_c1 = 8.4T and mu_0H_c2 = 48.9T, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60T and temperatures down to 0.1K reveal a H-T phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8K and 5.3K respectively. This complex phase diagram is not expected for a single-Q BEC system and therefore establishes AgVOAsO4 as a promising multi-Q BEC candidate capable of hosting exotic vortex phases.

The thermal conductivity of heavy-fermion superconductor ${\mathrm{Ce}}_{0.95}{\mathrm{Nd}}_{0.05}{\mathrm{CoIn}}_{5}$ was measured with a magnetic field rotating in the tetragonal $a\text{\ensuremath{-}}b$ plane. The thermal conductivity exhibits a step within the high-field low-temperature (HFLT) phase, which exists above 8 Tesla within the superconducting state, when field is rotated through the antinodal [100] direction of the superconducting $d$-wave order parameter. This anomaly indicates the presence of a third order parameter within the HFLT phase, similar to that of pure ${\mathrm{CeCoIn}}_{5}$. Therefore, the HFLT state with triply intertwined orders, i.e., superconducting $d$-wave, magnetic spin-density wave (SDW), and a putative superconducting $p$-wave pair-density wave (PDW), is robust against 5% Nd doping. The second magnetic phase within the superconducting state below 8 Tesla, a low-field SDW, displays a hysteresis in thermal conductivity as a function of field magnitude, which is also revealing the existence of a third order consistent with a $p$-wave PDW. Given different origins and properties of the HFLT and low-field SDW states, the presence of three intertwined orders in both phases is remarkable and of far-reaching consequences.

We report the influence of two-band superconductivity on the flux creep and the critical current densities of a MgB2 thin film. The small magnetic penetration depth of λ=50±10 nm at T=4 K is related to a clean π-band. We find a high self-field critical current density Jc, which is strongly reduced with applied magnetic field, and attribute this to suppression of the superconductivity in the π-band. The temperature dependence of the creep rate S (T) at low magnetic field can be explained by a simple Anderson–Kim mechanism. The system shows high pinning energies at low field that are strongly suppressed by high field.

We report the intrinsic superconducting parameters in a heavily overdoped Ba(Fe1-xCox)2As2 (x=0.14) single crystal and their influence in the resulting vortex dynamics. We find a bulk superconducting critical temperature of 9.8 K, magnetic penetration depth λab (0)=660 ± 50 nm, coherence length ξab (0)=6.4 ± 0.2 nm, and the upper critical field anisotropy γT→Tc ≈ 3.7. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. The intrinsic pinning energy plays an important role in the resulting vortex dynamics as compared with similar pinning landscape and comparable intrinsic thermal fluctuations. Fil: Kim, Jeehoon. Pohang University Of Science And Technology; Corea del Sur Fil: Haberkorn, Nestor Fabian. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Gofryk, K.. Oak Ridge National Laboratory; Estados Unidos Fil: Graf, M.J.. Oak Ridge National Laboratory; Estados Unidos Fil: Ronning, F.. Oak Ridge National Laboratory; Estados Unidos Fil: Sefat, A.S.. Oak Ridge National Laboratory; Estados Unidos Fil: Movshovich, R.. Oak Ridge National Laboratory; Estados Unidos Fil: Civale, L.. Oak Ridge National Laboratory; Estados Unidos

The ordering wave vector $\mathbf{Q}$ of a spin density wave (SDW), stabilized within the superconducting state of ${\mathrm{CeCoIn}}_{5}$ in a high magnetic field, has been shown to be hypersensitive to the direction of the field. $\mathbf{Q}$ can be switched from a nodal direction of the $d$-wave superconducting order parameter to a perpendicular node by rotating the in-plane magnetic field through the antinodal direction within a fraction of a degree. Here, we address the dynamics of the switching of $\mathbf{Q}$. We use a free-energy functional based on the magnetization density, which describes the condensation of magnetic fluctuations of nodal quasiparticles, and show that the switching process includes closing of the SDW gap at one $\mathbf{Q}$ and then reopening the SDW gap at another $\mathbf{Q}$ perpendicular to the first one. The magnetic field couples to $\mathbf{Q}$ through the spin-orbit interaction. Our calculations show that the width of the hysteretic region of switching depends linearly on the deviation of magnetic field from the critical field associated with the SDW transition, consistent with our thermal conductivity measurements. The agreement between theory and experiment supports our scenario of the hypersensitivity of the $Q$ phase on the direction of magnetic field, as well as the magnon condensation as the origin of the SDW phase in ${\mathrm{CeCoIn}}_{5}$.

The thermal conductivity measurement in a rotating magnetic field is a powerful probe of the structure of the superconducting energy gap. We present high-precision measurements of the low-temperature thermal conductivity in the unconventional heavy-fermion superconductor CeCoIn_{5}, with the heat current J along the nodal [110] direction of its d_{x^{2}-y^{2}} order parameter and the magnetic field up to 7 T rotating in the ab plane. In contrast to the smooth oscillations found previously for J∥[100], we observe a sharp resonancelike peak in the thermal conductivity when the magnetic field is also in the [110] direction, parallel to the heat current. We explain this peak qualitatively via a model of the heat transport in a d-wave superconductor. In addition, we observe two smaller but also very sharp peaks in the thermal conductivity for the field directions at angles Θ≈±33° with respect to J. The origin of the observed resonances at Θ≈±33° at present defies theoretical explanation. The challenge of uncovering their source will dictate exploring theoretically more complex models, which might include, e.g., fine details of the Fermi surface, Andreev bound vortex core states, a secondary superconducting order parameter, and the existence of gaps in spin and charge excitations.

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu2Ga2B. We demonstrate a feasible control both globally and locally of domain shapes and sizes by the external field as well as a smooth transition from single stripe to bubble domains, which opens the door to future applications based on magnetic domain tailoring.

A substance with controllable properties is valuable in many industrial applications. Scientists show how one such material---CeCoIn${}_{5}$---exists with three different superconducting and magnetic states intertwined together.

International audience; The interplay of magnetic and charge fluctuations can lead to quantum phases with exceptional electronic properties. A case in point is magnetically-driven superconductivity1,2, where magnetic correlations fundamentally affect the underlying symmetry and generate new physical properties. The superconducting wavefunction in most known magnetic superconductors does not break translational symmetry. However, it has been predicted that modulated triplet p-wave superconductivity occurs in singlet d-wave superconductors with spin-density-wave (SDW) order3,4. Here we report evidence for the presence of a spatially inhomogeneous p-wave Cooper pair-density wave in CeCoIn5. We show that the SDW domains can be switched completely by a tiny change of the magnetic field direction, which is naturally explained by the presence of triplet superconductivity. Further, the Q-phase emerges in a common magneto-superconducting quantum critical point. The Q-phase of CeCoIn5 thus represents an example where spatially modulated superconductivity is associated with SDW order.

We investigate partially disordered antiferromagnetism in ${\mathrm{CoCl}}_{2}$-2SC(${\mathrm{NH}}_{2}$)${}_{2}$, in which $ab$-plane hexagonal layers are staggered along the $c$ axis rather than stacked. A robust 1/3 state forms in applied magnetic fields in which the spins are locked, varying as a function of neither temperature nor field. By contrast, in zero field and applied fields at higher temperatures, partial antiferromagnetic order occurs, in which free spins are available to create a Curie-like magnetic susceptibility. We report measurements of the crystallographic structure and the specific heat, magnetization, and electric polarization down to $T=50\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{mK}$ and up to ${\ensuremath{\mu}}_{0}H=60\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The ${\mathrm{Co}}^{2+}\phantom{\rule{4pt}{0ex}}\mathbf{S}=3/2$ spins are Ising-like and form distorted hexagonal layers. The Ising energy scale is well separated from the magnetic exchange, and both energy scales are accessible to the measurements, allowing us to cleanly parametrize them. In transverse fields, a quantum Ising phase transition can be observed at 2 T. Finally, we find that magnetic exchange striction induces changes in the electric polarization up to $3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$, and single-ion magnetic anisotropy effects induce a much larger electric polarization change of $300\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$.

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation (BEC) and superfluidity, which have been tested experimentally in a variety of different systems. When bosons are interacting, disorder can destroy condensation leading to a so-called Bose glass. This phase has been very elusive to experiments due to the absence of any broken symmetry and of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (Br-doped dichloro-tetrakis-thiourea-Nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand-canonical ensemble; Br-doping introduces disorder in the hoppings and interaction strengths, leading to localization of the bosons into a Bose glass down to zero field, where it acquires the nature of an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the BEC (corresponding to a magnetically ordered phase) is marked by a novel, universal exponent governing the scaling on the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents the first, quantitative account of the universal features of disordered bosons in the grand-canonical ensemble., Comment: 13+6 pages, 5+6 figures; v2: Fig. 5 updated

Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d-electron materials, including the high-Tc copper-oxide superconductors, manganites, and most recently the iron-pnictide superconductors. The complexity generated by competing phases in these materials constitutes a considerable theoretical challenge-one that still defies a complete description. Here, we report a new manifestation of electronic inhomogeneity in a strongly correlated f-electron system, using CeCoIn5 as an example. A thermodynamic analysis of its superconductivity, combined with nuclear quadrupole resonance measurements, shows that nonmagnetic impurities (Y, La, Yb, Th, Hg and Sn) locally suppress unconventional superconductivity, generating an inhomogeneous electronic "Swiss cheese" due to disrupted periodicity of the Kondo lattice. Our analysis may be generalized to include related systems, suggesting that electronic inhomogeneity should be considered broadly in Kondo lattice materials., Comment: 27 pages

We use thermal conductivity to study a unique state in the high field corner of the superconducting phase of CeCoIn5. The spin-density-wave (SDW) magnetic order in this high field low temperature (HFLT) phase requires superconductivity for its very existence, and both magnetism and superconductivity disappear together at superconducting critical field H[Formula: see text]. We measured thermal conductivity of CeCoIn5 in a rotating magnetic field to study the nature of the HFLT phase. Our measurements reveal the presence of an additional order inside the HFLT phase that is intimately intertwined with the superconducting [Formula: see text]-wave and the SDW orders, which we identify as a superconducting [Formula: see text]-wave pair-density-wave (PDW). CeCoIn5 displays a hierarchy of interactions within the HFLT phase, where spin–orbit coupling orients the SDW, which then determines orientation of the secondary PDW component.

When subjected to pressure, the prototypical heavy-fermion antiferromagnet CeRhIn s becomes superconducting, forming a broad dome of superconductivity (SC) centred around 2.35 GPa (=P2) with maximal T c of 2.3 K. Above the superconducting dome, the normal state shows strange metallic behaviours, including a divergence in the specific heat and a sub-T-linear electrical resistivity. The discovery of a field-induced magnetic phase that coexists with SC for a range of pressures P ≤ P2 has been interpreted as evidence for a quantum phase transition, which could explain the non-Fermi-liquid behaviour observed in the normal state. Here we report electrical resistivity measurements of CeRhIn s under magnetic field at P2, where the resistivity is sub-T-linear for temperatures above T c (or T FL ) and a T 2 -coefficient A found below T FL diverges as H c2 is approached. These results are similar to the field-induced quantum critical compound CeCoIn 5 and confirm the presence of a quantum critical point in the pressure-induced superconductor CeRhIn 5 . Temperature-field phase diagram of CeRhIn 5 at 2.35 GPa.

Magnetic resonance force microscopy (MRFM) offers a means of performing local ferromagnetic resonance. We have studied the evolution of the MRFM force spectra in a continuous 20 nm thick cobalt-permalloy film as a function of lateral position. We clearly observed localized resonance signals originating from cobalt and permalloy. The spatial resolution in our experiments was determined to be better than 5 μm thus opening up the possibility to perform localized spectroscopic studies in continuous ferromagnetic samples.

The heavy-fermion antiferromagnet CeRhIn 5 exhibits a complex interplay between magnetism and unconventional superconductivity (SC) as a function of applied pressure and magnetic field. This interplay leads to a line of magnetic quantum-critical points within the superconducting state. A comparison of nuclear spin-relaxation measurements on CeRhIn 5 to those made on CeCoIn 5 , PuCoGa 5, and PuRhGa 5 suggests that SC and magnetism also may be related closely in these isostructural superconductors.

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ (with $x=0.32$). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripelike domains, followed by the formation of magnetic bubbles due to a field- and temperature-dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

We present specific heat and thermal conductivity of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical fieldH c2, measured with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase diagram and the first order nature of the superconducting phase transition at high fields close to a critical fieldH c2 indicate the importance of the Pauli limiting effect in CeCoIn5. In the same range of magnetic field we observe a second specific heat anomaly within the superconducting state, and interpret it as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a fieldH k below the superconducting critical field, which closely coincides with the low temperature anomaly in specific heat tentatively identified with the appearance of the FFLO superconducting state. The enhancement of thermal conductivity within the FFLO state calls for further theoretical investigations of the real space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.

We report specific heat and thermal conductivity measurements of the heavy fermion superconductor CeCoIn 5 in the vicinity of the superconducting critical field H c 2 , with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for field above 10 T, as evident from a sharp peak in specific heat and a jump in thermal conductivity, indicating the importance of the Pauli limiting effect in CeCoIn 5 . In the same range of magnetic field, we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a field H k below the superconducting critical field, which closely coincides with the low-temperature anomaly in specific heat, tentatively identified with the appearance of the FFLO superconducting state. Our results indicate that the thermal conductivity is enhanced within the FFLO state, and call for further theoretical investigations of the real-space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.

We report observation of the specific heat anomaly within the superconducting state of the heavy fermion CeCoIn5. It appears in the vicinity of the superconducting critical field Hc2, where the superconducting transition changes from second to first order, above 10 T for H || [1 1 0] and H || [1 0 0] , and above 4.7 T for H || [0 0 1] , and at temperatures on the order of 0.1Tc. We interpret the anomaly within the superconducting state as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov FFLO inhomogeneous superconducting state.

Following our recent discovery of superconductivity at ambient pressure in heavy fermion CeIrIn 5 and CeCoIn 5 , we studied the nature of the superconducting states in these compounds with specific heat, thermal conductivity, and NMR measurements. The low temperature specific heat in both CeIrIn 5 and CeCoIn 5 includes T 2 terms, consistent with the presence of nodes in the superconducting energy gap, indicating spin-singlet pairing. The thermal conductivity data present a T -linear term consistent with the universal limit (CeIrIn 5 ) and a low temperature T 3 variation in the clean limit (CeCoIn 5 ), also in accord with predictions for an unconventional superconductor with lines of nodes. An NMR study of CeCoIn 5 reveals suppressed spin susceptibility in the superconducting state, and the low temperature H c2 in CeCoIn 5 is Pauli limited, both indicating spin-singlet pairing. Taken together, our measurements point to d-wave superconductivity in this family of compounds.

The $A{R}_{2}{\text{O}}_{4}$ family ($R=\text{rare}$ earth) has recently been attracting interest as a new series of frustrated magnets, with the magnetic $R$ atoms forming zigzag chains running along the $c$ axis. We have investigated polycrystalline ${\mathrm{BaNd}}_{2}{\mathrm{O}}_{4}$ with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below ${T}_{N}=1.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$, and they can be indexed with a propagation vector of $\stackrel{P\vec}{k}=(0,1/2,1/2)$. The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double N\'eel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for $T=0.46\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. One possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical ${J}_{1}\ensuremath{-}{J}_{2}$ Ising chain with a double N\'eel ground state in zero field.

We present experimental results for the heavy-electron compound CeCu$_{4}$Ga which show that it possesses short-range magnetic correlations down to a temperature of $T = 0.1$ K. Our neutron scattering data show no evidence of long-range magnetic order occurring despite a peak in the specific heat at $T^{*} =1.2$ K. Rather, magnetic diffuse scattering occurs which corresponds to short-range magnetic correlations occurring across two unit cells. The specific heat remains large as $T\sim0$ K resulting in a Sommerfeld coefficient of $\gamma_{0} = 1.44(2)$ J/mol-K$^{2}$, and, below $T^{*}$, the resistivity follows $T^{2}$ behavior and the ac magnetic susceptibility becomes temperature independent. A magnetic peak centered at an energy transfer of $E_{\rm{c}}=0.24(1)$ meV is seen in inelastic neutron scattering data which shifts to higher energies and broadens under a magnetic field. We discuss the coexistence of large specific heat, magnetic fluctuations, and short-range magnetic correlations at low temperatures and compare our results to those for materials possessing spin-liquid behavior., Comment: 5 pages, 3 figures, Eq. 1 typo corrected from previous version

© 2014 Taylor & Francis. We report a molten metal flux technique to produce high purity specimens of URu2Si2. Magnetic susceptibility (M/H = x), heat capacity (C) and electrical resistivity (p) measurements show that the bulk properties of these crystals are similar to those that are produced by the Czochralski technique followed by solid-state electro-transport. In particular, we find residual resistivity ratios RRR = p300K /p0 as large as 220.

We report results of magnetic and transport measurements on single crystals of a new rare earth series RIr2Ga (R=La, Pr, Nd, Sm, and Gd through Tm). These compounds appear to crystallize in the triple-hexagonal Na3As structure with space group P63cm. Magnetic ordering temperatures show deviation from De Gennes scaling. These compounds are an interesting example of a quasi-2D structure in which there are anisotropic magnetic interactions.

Low temperature specific heat and thermal conductivity measurements on the ambient pressure heavy fermion superconductors CeIrIn5 and CeCoIn5 reveal power law temperature dependences of these quantities below T(c). The low temperature specific heat in both CeIrIn5 and CeCoIn5 includes T2 terms, consistent with the presence of nodes in the superconducting energy gap. The thermal conductivity data present a T-linear term consistent with the universal limit (CeIrIn5), and a low temperature T3 variation in the clean limit (CeCoIn5), also in accord with prediction for an unconventional superconductor with lines of nodes.

We report possible mixed valence behavior in the new ternary compounds CeIr 2 Ga and YbIr 2 Ga. These materials crystallize in the hexagonal Na 3 As structure of space group P6 3 cm. Magnetization, specific heat and transport measurements show reduced magnetic moments and the absence of magnetic order above 0.04 K. The quasi-2D structure of these compounds offers the possibility for investigating the role of spatial dimensionality on mixed valence phenomena.

Kondo insulator materials such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3, and SmB6, are 3d, 4f and 5f intermetallic compounds. At high temperatures they behave like metals but a gap Δ in the conduction band opens at the Fermi energy as the temperature is reduced. It has been proposed that the formation of the low-temperature gap is a consequence of the hybridization between the conduction band and the f-electron levels. If this is true, Kondo metal physics should be recovered when the gap is closed at high magnetic fields. We report here specific heat results of Ce3Bi4Pt3 in DC and pulsed magnetic fields up to 60 T. We see evidence for the reduction of the gap in 18 T and a rapid increase of the Sommerfeld coefficient C H /T T→0 in 30 T> H >40 T . Numerical results and the analysis of the data with the Coqblin–Schrieffer model prove a field-induced Kondo insulator-to-Kondo metal crossover.

The low temperature spin dynamics of the geometrically frustrated antiferromagnet Gd 3Ga 5O (12) (GGG) have been investigated using muon spin relaxation. No evidence for static order is seen down to a temperature of 25 mK or a few percent of the Curie-Weiss temperature. Instead there is a linear decrease in the Gd spin fluctuation rate below 1 K which extrapolates to a small but finite value of 2 GHz at zero temperature. In terms of the spin fluctuations the system appears essentially to remain dynamic at low temperatures (T>0.02 K) and magnetic fields up to 1.8 T.

Superconducting, magnetic, and non-Fermi-liquid-like ground states evolve in CeCu{sub 2+x}Si{sub 2} with small changes in the Ce/Cu ratio that leave the average crystal structure unchanged. We find, however, that the local atomic structure determined by neutron pair density function analysis for a superconducting sample with x=0.33 is fundamentally different from a nonsuperconducting sample with x=-0.08. The local lattice of the Cu-deficient sample exhibits long-range atomic order, while the Cu-rich sample is intrinsically disordered. Spin-lattice coupling in the disordered lattice produces an inhomogeneous magnetic state that allows superconductivity to develop and may contribute to observed non-Fermi-liquid effects. (c) 2000 The American Physical Society.

Superconductivity can be induced at high temperatures in Pb 2 Sr 2 RCu 3 O 8 (R, rare earth) by partially doping Ca 2+ for R 3+ . In order to understand the interplay between magnetism and superconductivity, the magnetic properties of the parent compounds, Pb 2 Sr 2 RCu 3 O 8, have been studied. The work presented here includes magnetic susceptibility and specific heat measurements on R=Dy and extends our previous studies on R=Ce, Pr, Tb, Ho and Er. Specific heat experiments suggest that the Dy ions order magnetically with an ordering temperature of 1.3 K. The magnetic susceptibility data are in good agreement with the susceptibility calculated using crystal-field parameters that are extrapolated from previous modeling of the R=Er and Ho analogs of this series.

Kondo insulator materials1—such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3 and SmB6—are 3d, 4f and 5f intermetallic compounds that have attracted considerable interest in recent years2,3,4,5. At high temperatures, they behave like metals. But as temperature is reduced, an energy gap opens in the conduction band at the Fermi energy and the materials become insulating. This contrasts with other f-electron compounds, which are metallic at all temperatures. The formation of the gap in Kondo insulators has been proposed to be a consequence of hybridization between the conduction band and the f-electron levels6,7, giving a ‘spin’ gap. If this is indeed the case, metallic behaviour should be recovered when the gap is closed by changing external parameters, such as magnetic field or pressure. Some experimental evidence suggests that the gap can be closed in SmB6 (refs 5, 8) and YbB12 (ref. 9). Here we present specific-heat measurements of Ce3Bi4Pt3 in d.c. and pulsed magnetic fields up to 60 tesla. Numerical results and the analysis of our data using the Coqblin–Schrieffer model demonstrate unambiguously a field-induced insulator-to-metal transition.

Competing interactions in the presence of coupled spin, charge, and lattice degrees of freedom in heavy-fermion materials lead to a near degeneracy of ground states in some systems. A small perturbation in unit-cell volume or composition subsequently can produce a qualitative change in the ground state, for example, from magnetically ordered to superconducting, with, in some instances, the appearance of a non-Fermi-liquid (NFL) state near their boundary. We have studied two heavy-fermion materials, CeCu 2 Si 2 and CeRh 2 Si 2 , which illustrate these behaviors. Measurements of the atomic structure of CeCu 2+ x Si 2 as a function of x suggest that structural inhomogeneity may influence the preferred ground state and the existence of NFL behavior in this material. Thermal expansion measurements on CeRh 2 Si 2 as a function of pressure reveal the evolution of spin-lattice coupling as the balance between RKKY and Kondo interactions is tuned by small volume changes.

CeCoIn5 undergoes a phase transition within the superconducting (SC) state. A new superconducting state occupies a high field–low temperature (HFLT) corner of the SC state in the H–T plane, and is a strong candidate for realization of the spatially inhomogeneous Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting state predicted theoretically in 1960s. We present specific heat data on Cd- and Hg-doped CeCoIn5 that shows that the HFLT phase is suppressed by minute amount of impurities, few hundred parts per million. At this concentration, the average distance between impurities is close to twice the superconducting coherence length ξ, suggesting that ξ is the characteristic length of the HFLT state. We conclude that our investigations support the superconducting, posibly FFLO, origin of the HFLT state.

PHYSICAL REVIEW B VOLUME 60, NUMBER 6 1 AUGUST 1999-II Structure and physical properties of the quaternary Remeika-phase compound Yb 5 Pt 6 In 16 Bi 2 E. G. Moshopoulou, M. F. Hundley, R. Movshovich, J. D. Thompson, J. L. Sarrao, * and Z. Fisk * Los Alamos National Laboratory, Los Alamos, New Mexico 87545 E. Felder, M. Chernikov, D. Pushin, and H. R. Ott Laboratorium fu¨r Festko¨rperphysik, Eidgeno¨ssische Technische Hochschule–Ho¨nggerberg, 8093 Zu¨rich, Switzerland 共Received 12 January 1999兲 We report the synthesis and characterization of an indium-rich quaternary Yb-Pt-In-Bi compound that is similar to the well-known rare-earth transition-metal stannide Remeika-phase compounds. In addition to structural/composition studies identifying the compound as Yb 5 Pt 6 In 16 Bi 2 , we have made dc and ac magnetic susceptibility, specific heat, resistivity, magnetoresistance, Hall effect, and thermoelectric power measurements on this material. It appears that this Remeika-phase compound 共structure type Tb 5 Rh 6 Sn 18 ) is moderately disordered. Crystal-field effects influence the temperature dependence of the dc magnetic susceptibility, the specific heat, the resistivity, and the thermoelectric power. A cusp in the ac magnetic susceptibility peaks near 300 mK, close in temperature to where the specific heat divided by temperature C p (T)/T also reaches a maximum. The susceptibility feature is frequency dependent and suggestive of a disorder-induced spin-glass transition rather than conventional magnetic order. 关S0163-1829共99兲03830-8兴 INTRODUCTION Since being discovered in the early 1980s, 1–3 Remeika- phase compounds 共tin-rich rare-earth transition-metal stan- nides兲 have attracted much attention, especially because of the coexistence of magnetism and superconductivity that is observed in some members of this family of compounds. 4 Although distorted versions of each exist, 5,6 the materials can be divided into two groups. The first are those derived from a face-centered-cubic structure with lattice constant a ⬇9.7 A, such as R 3 M 4 Sn 13 , often written SrR 3 M 4 Sn 12 be- cause of the multiplicity of Sn sites, where R is a rare earth and M is a transition-metal element. The more complicated group are those derived from a face-centered-cubic structure or with a⬇13.7 A, for example R 5 M 6 Sn 18 Sn 1⫺x R x R 4 M 6 Sn 18 . Both classes of compounds form with many different rare earths and transition metals. 4 We have found that it is also possible to synthesize indium-rich com- pounds in the same structure that seem to share many of the physical properties of these stannides. Here, we report the observation of the larger-cell structure in an Yb-Pt-In-Bi quaternary phase. We also have observed the smaller-cell phase in rare earth-platinum-indium ternaries. 7 The only pre- viously reported group of such compounds that do not con- tain Sn are the rare-earth transition-metal germanides, dis- cussed by Braun. 8 SYNTHESIS AND STRUCTURAL CHARACTERIZATION In an attempt to grow single crystals of Yb-Bi-Pt phases from an indium flux, 9 we have grown single crystals of a quaternary phase Yb-Pt-In-Bi compound. The crystals grow from an approximately 10 molar % solution of the composi- tion YbBiPt in excess In. The synthesis is made in alumina crucibles encapsulated in evacuated silica, by slow cooling from 1100 °C. Multi-faceted crystals with faces up to 1 cm in PRB 60 lateral dimension grow from these melts. They have an ap- pearance and morphology very similar to what is found in the growth of the Remeika stannides from excess Sn. The crystals are clearly quaternary phases as it is not possible to synthesize the material from melts that exclude any of the constituent elements 共in particular Bi兲. Furthermore, it has not proved possible to synthesize the Yb-Pt-In analogue of the 9.7-A fcc stannide, although such a phase can be pro- duced with light rare earths, for example Ce 3 Pt 4 In 13 . 7 The symmetry of the Yb-Pt-In-Bi crystals was established from precession photographs using Mo K ␣ radiation. The photographs display Laue symmetry m3m and reflection conditions hkl: h⫹k,h⫹l,k⫹l⫽2n, 0kl: k,l⫽2n, hhl: h ⫹l⫽2n, h00: h⫽2n, consistent with the space groups F432, F-43m, and Fm-3m. The F cell parameter, deter- mined from the precession photographs, is a⬇13.8 A. Crushed crystals also were examined by powder x-ray dif- fraction, using a Siemens diffractometer and Cu K ␣ radiation, in order to determine accurately the cell constant. The final lattice constant, refined by the least squares method using the absolute ␪ angle of 12 high angle reflections, was a Quantitative x-ray microanalysis was performed by apply- ing the Cliff-Lorimer ratio technique 10 to data obtained with a Phillips CM 300 transmission electron microscope 共TEM兲 operating at 300 kV and equipped with a Kevex x-ray energy-dispersive 共XEDS兲 analysis system. The Cliff- Lorimer 共or sensitivity兲 factors, k, used in the XEDS analy- sis, were determined from analysis of the stoichiometric standard compounds YbBiPt and YbInCu 4 . TEM samples of Yb-Pt-In-Bi and of the standard compounds YbBiPt and YbInCu 4 were prepared by crushing single crystals of each compound in anhydrous methanol, dispersing them onto a 1000-mesh TEM grid, and immediately inserting them into the microscope, in order to avoid potential contamination. ‘‘Beam transparent’’ areas along the edges of many crystal- ©1999 The American Physical Society